It's a powerful demonstration of the role of meta-analysis and cumulative science in the context of causal inference research. Often, and this isn't the fault of those conducting the research, people tend to accept single studies as definitive proof of a causal relationship.

No single study, no matter how well-designed or identified, proves anything. Science at its core is Bayesian - it's a collaborative effort where bodies of work prove or show things, and studies only nudge us some distance one way or another.